There’s a startling cheating secret behind the divorce of ABC’s “20/20” newsgal Elizabeth Vargas. Elizabeth, 52, – who was once Michael Douglas’ girlfriend –is splitting from “Walking in Memphis” songwriter Marc Cohn, 55. They have two sons – Zachary, 11, and Samuel, 8.

News of the divorce broke in late August, while Elizabeth returned to rehab for alcohol addiction. In March, word had leaked that Marc allegedly cheated on her while she was in an earlier rehab stint.

He reportedly had an affair with Flywheel Sports co-founder Ruth Zukerman, and a source said: “It was disgusting that he was with Ruth while Elizabeth was in rehab.”

A friend told The National ENQUIRER: “It’s terrible – Elizabeth hates himfor what he did, but she still needs him to look after their kids while she’s in rehab.”